1. Field of the Invention
The present invention disclosed in this specification relates to a semiconductor device having a thin film transistor such as formation of a gate insulating film of a thin film transistor or a protective film of a gate electrode.
2. Description of the Related Art
A thin film transistor is widely known as a switching element used in an active matrix display device. In a manufacturing process of a thin film transistor, a CVD method or a thermal oxidation method has conventionally been employed in general so as to form an insulating film.
However, there has been a problem in that a silicon oxide film formed by a CVD method is inferior to a silicon oxide film obtained by a thermal oxidation method in film quality, such that the silicon oxide film formed by a CVD method lacks in density, contains much impurities such as carbon, and suffers a damage due to plasma (plasma damage).
On the contrary, in order to efficiently form a silicon oxide film having a predetermined thickness with high quality by a thermal oxidation method, it is required to oxidize silicon in an oxygen atmosphere at a temperature of 800° C. or more. Thus, in the case of employing a thermal oxidation method in forming a gate insulating film of a thin film transistor, a glass substrate typified by non-alkali glass cannot be used and a quartz substrate which is more expensive than the glass substrate is forced to be used.
In addition, when thermal oxidation is performed to silicon having a corner portion, a thickness of a silicon oxide film formed over the corner portion of the silicon becomes thinner in some cases, compared with a thickness of a silicon oxide film formed over a roughly plane portion of a top surface of the silicon. This is because oxidation is suppressed due to stress caused by a shape of the corner portion.
In the future, it is required to make a thinner gate insulating film than ever before in accordance with more miniaturization of a thin film transistor. For example, although a gate insulating film is conventionally formed with a thickness of 100 nm or more, it is required to be formed with a thickness of several tens of nm. However, in the case of using a silicon oxide film formed by the above-described conventional method as a gate insulating film, the thinner the thickness thereof becomes, the more the amount of leakage current flowing between a semiconductor film including a channel formation region and a gate electrode via the thin silicon oxide film is increased. Further, in a case where a silicon oxide film formed as a gate insulating film does not have a uniform thickness and locally has a thin portion, there is a possibility of generating leakage current via the thin portion.
As a material for forming the gate insulating film, silicon oxynitride (denoted by SiOxNy, note that x>y) is sometimes used instead of silicon oxide. However, heat treatment at a high temperature exceeding a strain point of a glass substrate is required to form the silicon oxynitride film by heat treatment in an atmosphere such as N2O which is capable of performing nitridation.
Recently, a method of forming a gate insulating film of a field effect transistor for an LSI with a plasma treatment apparatus which is capable of performing plasma oxidation and plasma nitridation has been focused. For example, it is disclosed in Reference 1 that a silicon nitride film to be a gate insulating film is formed over a semiconductor layer by directly reacting nitrogen activated by plasma excitation with silicon of the semiconductor layer (Reference 1: Japanese Patent Laid-Open No. 2004-319952). However, according to Reference 1, disclosed are only an example of using an SOI (Silicon On Insulator) substrate and a point that the semiconductor layer may be a bulk semiconductor substrate, and an attempt to form a gate insulating film of a thin film transistor with an apparatus capable of performing plasma oxidation and plasma nitridation is not disclosed.